Designers and inventors have sought to develop a light modulator which can operate alone or together with other modulators. Such modulators should provide high operating speeds (KHz frame rates), a high contrast ratio or modulation depth, have optical flatness, be compatible with VLSI processing techniques, be easy to handle and be relatively low in cost. Two such related systems are found in U.S. Pat. Nos. 5,311,360 and 5,841,579 which are hereby incorporated by reference.
According to the teachings of the '360 and '579 patents, a diffractive light modulator is formed of a multiple mirrored-ribbon structure. An example of such a diffractive light modulator 10 is shown in FIG. 1. The diffractive light modulator 10 comprises elongated elements 12 suspended by first and second posts, 14 and 16, above a substrate 20. Thc substrate 20 comprises a conductor 18. In operation, the diffractive light modulator 10 operates to produce modulated light selected from a reflection mode and a diffraction mode.
FIGS. 2 and 3 illustrate a cross-section of the diffractive light modulator 10 in a reflection mode and a diffraction mode, respectively. The elongated elements 12 comprise a conducting and reflecting surface 22 and a resilient material 24. The substrate 20 comprises conductor 18.
FIG. 2 depicts the diffractive light modulator 10 in the reflection mode. In the reflection mode, the conducting and reflecting surfaces 22 of the elongated elements 12 form a plane so that incident light I reflects from, the elongated elements 12 to produce reflected light R.
FIG. 3 depicts the diffractive light modulator 10 in the diffraction mode. In the diffraction mode, an electrical bias causes alternate ones of the elongated elements 12 to move toward the substrate 20. The electrical bias is applied between the reflecting and conducting surfaces 22 of the alternate ones of the elongated elements 12 and the conductor 18. The electrical bias results in a height difference between the alternate ones of the elongated elements 12 and non-biased ones of the elongated elements 12. A height difference of a quarter wavelength λ/4 of the incident light I produces maximum diffracted light including plus one and minus one diffraction orders, D+1 and D−1.
FIGS. 2 and 3 depict the diffractive light modulator 10 in the reflection and diffraction modes, respectively. For a deflection of the alternate ones of the elongated elements 12 of less than a quarter wavelength λ/4, the incident light I both reflects and diffracts producing the reflected light R and the diffracted light including the plus one and minus one diffraction orders, D+1 and D−1. In other words, by deflecting the alternate ones of the elongated elements 12 less the quarter wavelength λ/4, the diffractive light modulator 10 produces a variable reflectivity.
In WDM (wavelength division multiplex) optical communication, multiple component wavelengths of light each carry a communication signal. Each of the multiple component wavelengths of light form a WDM channel. A dynamic gain equalizer (DGE) can he used for WDM signal management. A variety of dynamic equalization techniques have been advanced, which seek to equalize component signals in a WDM system. Most rely on some spectral multiplexer/de-multiplexer component, followed by an electrically-controllable variable optical attenuator which can operate on the de-multiplexed channels (or possibly a band of channels). Diffractive light modulators arc often used as the variable optical attenuator within a DGE. Each channel is directed to a corresponding portion of the diffractive light modulator. To maximize space, each channel partially overlaps an adjacent channel as the channels impinge the diffractive light modulator. Overlapping channels is useful to minimize the number of required ribbons. If channels are not overlapped, then the optical path has to be increased, which leads to a larger optical package.
What is needed is a method and apparatus for improving the accuracy of a DGE that utilizes overlapping channels. What is also needed is a method and apparatus for reducing the computational power required for DGE that utilizes overlapping channels.